388 PROCESSES OF LIFE REVEALED BY THE MICROSCOPE. 



the other arm remaining at rest. Then the two sides of the nervous 

 system — that is, the pairs of nerves to the arms with their ganglia and 

 a segmeut of the myel (spinal cord) — were removed and treated with 

 fixing agents, and carried through all the processes necessary to get 

 thin sections capable of accurate study with the microscope. Finally 

 upon the same glass slide are parts of the nervous system fatigued 

 even to exhaustion, and corresponding parts of the same animal which 

 had "been at rest. Certainly if the nervous substance shows the result 

 or processes of its action the conditions are here perfect. Fatigued 

 nerve cells are side by side with those in a state of rest. The appear- 

 ances are clear and unmistakable. The nucleus has markedly decreased 

 in size in the fatigued cells and possesses a jagged, irregular outline 

 in place of the smooth, rounded form of the resting cells. The cell 

 substance is shrunken in size and possesses clear, scattered spaces, or 

 a large, clear space around the nucleus. 



If the nervous substance was not fixed at once but remained in the 

 living animal for twelve to twenty-four hours in a state of repose, the 

 signs of exhaustion disappeared and the two sides appeared alike. By 

 studying preparations made after various periods of repose all the 

 stages of recovery from exhaustion could be followed. 



For possible changes in normal fatigue, sparrows, pigeons, and swal- 

 lows, and also honeybees, were used. For example, if two sparrows or 

 two honeybees as nearly alike as possible were selected, the nervous 

 system of one being fixed in the morning after the night's rest and that 

 of the other after a day of toil, the changes in the cells of the brain of 

 the honeybee or sparrow and in the spinal ganglia of the sparrow were 

 as marked as in case of artificial fatigue. After prolonged rest, then, 

 the nerve cells are, so to speak, charged; they are full and ready for 

 labor; but after a hard day's work they are discharged — shrunken and 

 exhausted. (See Plate XIV.) 



There is one more step in this brilliant investigation. If in the morn- 

 ing, after sleep and rest, animals and men are full of vigor, and in the 

 evening are weary and exhausted, how like is it to the beginning and 

 end of life? In youth so overflowing with vigor that to move, to act, 

 is x>leasure, and continued rest a pain ; but in the evening of life a warm 

 corner and repose are what we try to furnish those whose work is done. 

 How is this correlated in the cells of the nervous system with the states 

 of rest and fatigue? With a well-nourished child which died from one 

 of the accidents of birth the nerve cells showed all the characters of 

 cells at rest and fully charged. In a man dying naturally of old age the 

 cells showed the shrunken nuclei and all the appearances of exhausting 

 fatigue. In the one was the potentiality of a life of vigorous action ; the 

 other showed the final fatigue — the store of life energy had been dissi- 

 pated, and there was no recovery possible. 



For the animals that possess an undoubted nervous system probably 

 all would admit that there is some sort of nervous action corresponding 



